We demonstrate the performance of thin film transistor devices based on a solution processable organic semiconductor
with a field effect mobility of up to
2.5cm<sup>2</sup>V<sup>-1</sup>s<sup>-1</sup>. The performance of the material is demonstrated in a top gate, bottom
contact device architecture operational in air without the requirement for device encapsulation. From a device
performance aspect, we also highlight the influence that contact resistance has on the mobility.
High performance short channel OTFTs with field effect mobilities greater than 1cm<sup>2</sup>V<sup>-1</sup>s<sup>-1</sup> have been developed for
OLED driver applications incorporating soluble crystalline semiconductor materials. We highlight the impact of contact
resistance on the mobility in these devices and show by functionalising both the source and drain contacts and channel
regions in a top gate bottom contact device architecture that the mobility can be significantly improved. Our approach
also includes the optimisation of solvent selection from which the semiconductor material is deposited in order to
enhance crystalline domain formation.
The phase behaviour of poly(9,9-dioctylfluorene-co-bithiophene) semiconducting polymer, (F8T2) in top gate thin film transistor device structures fabricated using inkjet printing is investigated. The source, drain and gate electrodes are patterned by inkjet printing from a solution of a conducting polymer, poly(3,4-ethylene dioxythiophene) (PEDOT) doped with poly(styrene sulfonic acid) (H. C. Starck), and a polymer layer is used as the dielectric.
At room temperature, the as-spun semiconductor films exhibit an isotropic, amorphous phase. Field effect mobilities of more than 4 x 10<sup>-3</sup> cm<sup>2</sup> / Vs, and on / off current ratios greater than 10<sup>5</sup> are observed. Upon annealing at elevated
temperatures, crystalline, and liquid crystalline phases are exhibited. The crystalline domains are identified by polarised
optical and atomic force microscopy. We investigate the crystallinity as a function of the annealing temperature. The order in the material is found to correlate well to the field effect mobility in the TFT device structure. The results of TFTs fabricated using inkjet printing to deposit the semiconductor film are also shown.
Recently there has been renewed interest in the grating alignment of liquid crystals because of its application in bistable nematic displays. In this paper, gratin and photoinduced liquid crystal alignment techniques based on excimer laser exposure of thin polyimide films are discussed. Gratings are etched into the alignment film using a KrF laser illuminated through a phase mask. These give homogeneous liquid crystal alignment with the liquid crystal directors aligned along the grooves of the grating. The observed azimuthal anchoring strength is compared with that predicted using Berreman theory. No pretilt is observed because of the grating symmetry. When a polarized excimer laser beam is incident on the film with a fluence below that required for ablation, an anisotropy is created photochemically by selective depletion of the polymer chains. Exposure of the polyimide with elliptically polarized light at non-normal incidence gives pretilted alignment. Grating etching followed by photoinduced alignment can be used to obtain pretilted grating alignment with a pretilt angle of 3 degrees.
Gratings etched into polyimde thin layers are used to align nematic liquid crystals. The gratings are prepared by illumination of a 1.1 micrometer period phase mask with a KrF excimer laser at 248 nm. Fluences of 87 mJ cm<SUP>-2</SUP> and 128 mJ cm<SUP>-2</SUP> with one and two shots were used to ablate the gratings. Modelling of the fluence distribution behind the phase mask predicts a grating period equal to that of the phase mask and this is found experimentally. The amplitudes of the gratings are obtained from diffraction using a HeNe laser and are between 100 nm and 150 nm deep. The alignment layers are used in twisted nematic cells and the azimuthal anchoring energy is measured as a function of grating fabrication conditions. Anchoring energies of the order of 10<SUP>-5</SUP> J m<SUP>-2</SUP> are found in agreement with the Berreman theory.